1. Field of the Art
The present invention relates to a hydraulic control apparatus for controlling a hydraulically-operated power transmitting system used in an automotive vehicle, having a continuously variable transmission (hereinafter referred to as "CVT").
2. Related Art Statement
A power transmitting system for a vehicle including a CVT is provided with an auxiliary transmission for changing a direction of drive of the vehicle and for improvement in drivability. For example, a power transmitting system is disclosed in Japanese Patent Applications Nos. 58-144985 (laid open under Publication No. 60-37455) and 59-12017 (laid open under Publication No. 60-157554), which includes a CVT, and an auxiliary or secondary transmission connected in series to the CVT. In such a transmitting system, the auxiliary transmission is substantially always held in its high-range position for improved fuel economy while an operator controlled manual shift lever is set in a Drive position for an ordinary driving mode. While the manual shift lever is set in a Low position, the auxiliary transmission is substantially always placed in its low-range position for obtaining enough power for running up a slope, or for obtaining an engine-braking effect. For shifting up and down the auxiliary transmission between the low-range and high-range positions, the hydraulic system for the transmission uses a shift valve which is controlled by means of a solenoid valve. However, there is a possiblity that the solenoid valve may be operated due to an erroneously generated electric control signal and the auxiliary transmission may be shifted from its high-range position to its low-range position while the vehicle is running at a high speed in the Drive position. In such event, a shift-down shock is given to the auxiliary transmission.